Anisotropy of the dielectric function in YBa2Cu3O6.

Abstract

Experimental data for the anisotropy of the dielectric tensor {epsilon}={epsilon}{sub 1}+i{epsilon}{sub 2} in YBa{sub 2}Cu{sub 3}O{sub 6} are presented and interpreted. The spectra in the range between 1.7 and 5.2 eV were obtained by rotating-analyzer ellipsometry using a single crystal. It is found that the component of {epsilon}{sub 2} in the {ital ab} plane ({epsilon}{sub 2}{sup {ital a}}) displays a sharp peak at 4.1 eV, while the component along the {ital c} axis ({epsilon}{sub 2}{sup {ital c}}) has a less pronounced peak at 3.8 eV, a small peak at 4.2 eV, and a broad maximum near 5 eV. Similar structures were found in the interband spectra calculated using the local-density approximation. The 4-eV optical structure is caused by nearly parallel initial- and final-state bands whose wave-functions are located, respectively, in the O-Cu-O dumbbells and in the BaO-Cu-OBa triple layers. Intraionic 3{ital d}{r arrow}4{ital p} transitions in dumbbell Cu contribute 90% of the optical matrix elements.